In an electric power system, for example, in high and medium-voltage power networks, substations include primary devices such as electrical cables, lines, bus bars, switches, power transformers and instrument transformers, which are generally arranged in switch yards and/or bays. These primary devices are operated in an automated way via an SA system. The SA system includes secondary devices, including Intelligent Electronic Devices (IEDs), which are responsible for the protection, control and monitoring of the primary devices. IEDs for SA or process control are embedded devices configured to execute protection functions for the primary equipment. To provide for failing protection functions, in high-voltage power networks, redundant protection is implemented locally, that is, inside the substation.
According to EP-A 1976177, in SA systems, the mean time to repair is reduced by means of remote re-configuration and start-up of a replacement or spare IED, leaving some more hours for the maintenance personnel to repair an inactive or faulty IED. The time required for the actual repair is irrelevant for the system availability as long as it is short enough compared to the IED failure rate. Therefore, the remote configured spare IED leads to nearly the same availability as a hot, standby configuration, but without the need for doubling all the essential IEDs, since only one spare online IED is needed for each set of IEDs of the same type connected to the same station bus and process bus.
Nevertheless, in the context of medium-voltage substations, protection functions for the primary equipment are usually not redundant. The main reason for avoiding redundancy is cost efficiency. Indeed, the known manner to provide protection function redundancy (such as in high voltage substations) requires duplication of the hardware devices, such as IEDs, which execute the protection functions. From a cost point of view, such an approach almost doubles the price of the protection part of the SA system, because of the redundant hardware as well as the engineering part spent on setting up the redundant system.